[3H]-2-Nitroimipramine was prepared and its binding to the high affinity binding site for [3H]-imipramine in brain and platelets was characterized. This ligand had high affinity and dissociated extremely slowly at 0-4 C, indicating its suitability for labelling and purification of this binding site. Drug discrimination studies of (+)-and (-)-1-(1-phenylcyclohexyl)-3-methylpiperidine (PCMP) in pigeons revealed that the (+)-isomer was about 5-fold more potent than the (-)-isomer in producing ketamine appropriate responding. In another demonstration of stereospecificity, (+)-isomer was equipotent with PCP in producing changes in heart rate, blood pressure and suppression of plasma prolactin release in the rat while the (-)-isomer was essentially ineffective.